Robust Laser Technology Will Enable Satellite to Measure Greenhouse Gas

The MERLIN satellite, a collaboration between DLR RfM in Germany and CNES in France, will use a radar-like laser system to measure the methane concentration in Earth’s atmosphere. The satellite’s Integrated Path Differential Absorption (IPDA) lidar will send laser light to Earth’s surface and analyze the backscattered signal.

Ferdinand-Braun-Institut (FBH) developed and manufactured laser diode benches (LDBs) for the satellite’s lidar system. Six of these space-qualified LDBs have been integrated into three pump modules from the Fraunhofer Institute for Laser Technology (ILT). The modules generate the necessary pump energy for the oscillator of a Nd:YAG solid-state laser and are integrated into the lidar system.

Laser diode bench from the Ferdinand-Braun-Institut before mounting into the pump module. Courtesy of FBH/schurian.com.

The solid-state laser serves as the light source for a tunable optical parametric oscillator that generates double pulses with different wavelengths in the infrared range around 1.6 μm. One of these pulses is strongly absorbed by methane, and the other is not. The methane content can be determined from the ratio of intensities of the backscattered light.

Each LDB generates a pump power of over 60 W in double pulses with a repetition rate of 20 Hz and 150-μs pulse width. Laser diode minibars in the LDB ensure efficient light generation. Thanks to the fast axis collimation lenses integrated in the LDB, the laser beam can be coupled into an optical fiber with a minimum of loss. Two of these LDBs are integrated into each module and provide a joint pump power of 120 W.

The diode laser, fast axis collimation, and LDB technology were tested at FBH and confirmed as being suitable for space applications. Subsequently, the laser diode benches were qualified by the European Space Research and Technology Center (ESTEC) in the Netherlands. The extensive life cycle tests performed by ESTEC showed that the power degrades only minimally even after a long operating time of more than four billion pulses. The scientific team is therefore confident that the MERLIN measuring system will function failure-free even under space conditions.

As part of the MERLIN satellite project, the Fraunhofer ILT is developing the beam source of the laser transmitter under contract to Airbus DS GmbH. The satellite is scheduled to be launched in 2025,

The MERLIN mission aims to better understand the effect of the greenhouse gas methane on global warming. The satellite’s detailed investigation of this important source of global warming could be a milestone in European climate research.